The present invention relates to a switching power supply wherein a current detection resistor is inserted between a switching element such as an FET and a primary side ground level and a current flowing into the switching element is limited based on a voltage generated between the both terminals of the current detection resistor.
In an RCC type switching power supply, the configuration shown in FIG. 2 is proposed as a conventional technique which can prevent abnormal heat etc. even in the case of the short-circuit of a primary coil or a switching element. That is, supposing that a zener diode D5 is eliminated, when a short-circuit of a primary coil L1 or an internal short-circuit of an FET 5 occurs, an over-current flows through a current detection resistor R1. This over-current causes an open breakage of the current detection resistor R1 before fusing of a fuse 3. When the open breakage of the current detection resistor R1 occurs, a high voltage exceeding 100 volt is applied to the base of a control transistor Q2 through a diode D1. As a result, abnormal heat occurs at the control transistor Q2 and so a firing may occur before melting of the fuse 3.
Thus, although it is desirable to use as the current detection resistor R1 a cemented resistor which is quite low in possibility of causing the open breakage, the cemented resistor is expensive. Therefore, conventionally, a metal film resistor which element cost is cheap is used as the current detection resistor R1, and the zener diode D5 is connected in parallel with the current detection resistor R1. In this case, even if the primary coil L1 or the zener diode D5 is short-circuited and the open breakage of the current detection resistor R1 occurs, the base voltage of the control transistor Q2 is limited to the zener voltage of the zener diode D5. Thus, the abnormal heat of the control transistor Q2 is suppressed. Therefore, since the fuse 3 melts before the breakage of the zener diode D5, the heat generation is stopped thereafter (first conventional technique).
Further, in a switching power supply, a technique for preventing the breakage etc. of a load caused by an over-voltage is proposed in the Unexamined Japanese Patent Application Publication No. Hei11-149320. That is, in this technique, an over-current fusing type resistor is inserted between a primary side positive power supply and a switching element. A thyristor is provided whose one end is coupled at a connection point between the switching element and the over-current fusing type resistor and the other terminal is grounded. Further, an over-voltage detection circuit is provided which shifts the thyristor from an off state to an on state when an output voltage exceeds a predetermined value. Thus, when the output voltage increases to an abnormal voltage due to an abnormality of the switching element etc., the thyristor becomes in an on state. As a result, the output voltage reduces to almost 0 volt. Further, a current sufficient for fusing flows through the over-current fusing type resistor. Thus, when the output voltage becomes a high value due to an abnormality of the switching element etc., since the over-current fusing type resistor fuses in a short time, a load is protected from the breakage due to an over-voltage (second conventional technique).
In a technique proposed in the Unexamined Japanese Patent Application Publication No. Hei9-266669, a fuse resistor is provided independently from a smoothing circuit constituted by a resistor and a smoothing capacitor. One terminal of the fuse resistor is coupled to the smoothing circuit and the other terminal thereof is coupled to a primary coil. Thus, a rush current to the smoothing capacitor at the time of turning-on of the power supply does not flow into the fuse resistor. That is, there is no fear that the fuse resistor melts by the rush current. Therefore, the fusing current value of the fuse resistor may be set to a value necessary for the protection of the load without taking the influence of the rush current into consideration, the reliability with respect to the protection for the load can be improved (third conventional technique).
However, when the first conventional technique is employed, the zener diode D5 is required. Further, the zener diode D5 is required to have a durability sufficient for not causing the breakage of the element itself until the fuse 3 melts. Thus, since the zener diode D5 is required to be an element with a large rated watt, the zener diode D5 is expensive.
When considering the second conventional technique in a view point of eliminating the problem of the first conventional technique, that is, when considering the second conventional technique in a view point of preventing without using an expensive element the generation of the abnormal heat at the time of the open breakage of the current detection resistor R1 coupled between the source of the FET 5 and the primary side ground level, this technique is difficult to be applied to the switching power supply even only in a view point of using the fuse resistor since the fuse resistor is an expensive element. Further, since the thyristor is required to be provided additionally, the cost of the parts for the switching power supply increases further.
The third conventional technique is formed to have a circuit configuration intended not to flow the rush current into the fuse resistor. That is, when considering the third conventional technique in a view point of eliminating the problem of the first conventional technique, since the third conventional technique differs in the basic configuration of the switching power supply, the third conventional technique can not prevent the generation of the abnormal heat at the time of the open breakage of the current detection resistor R1 coupled between the source of the FET 5 and the primary side ground level. Further, the third conventional technique is arranged to protect the load based on the rated power ratio of the resistor itself which is configured to easily case the open-breakage thereof.
The invention is made in order to solve the aforesaid problems of the conventional techniques an object of the invention is to provide a switching power supply which can prevent abnormal heat of a control transistor at the time of the short-circuit of a primary winding or an FET without using a zener diode which is an expensive element by differentiating the current values from which two resistors start the open-breakage, respectively; which can easily determine resistance values and rated watts of respective resistors used as a breaking resistor and a current detection resistor which are inserted in a path from the output point of a primary side positive power supply to the drain of the FET; and which can suppress the dissipation power of the breaking resistor to a minimum value even when surely causing the open-breakage of the breaking resistor without causing the open-breakage of the current detection resistor.
Further, another object of the invention is to provide a switching power supply wherein the breaking resistor is inserted at an arbitrary portion of the path from the output point of the primary side positive power supply to the drain of the FET and the breaking resistor is set to be open-broken more easily than the current detection resistor based on the relation of the resistance values and the rated watts between the breaking resistor and the current detection resistor, whereby abnormal heat of the control transistor can be prevented at the time of the short-circuit of the primary coil or the FET without using a zener diode which is an expensive element.
Furthermore, in addition to the aforesaid objects, still another object of the invention is to provide a switching power supply which can easily determine resistance values and rated watts of respective resistors used as the breaking resistor and the current detection resistor by forming each of the breaking resistor and the current detection resistor by a metal film resistor.
In order to solve the aforesaid problems, the switching power supply according to the invention is arranged in a manner that in the switching power supply including:
a transformer around which a primary coil and a secondary coil are wound, one terminal of the primary coil being introduced to a primary side positive power supply;
a switching element having a current input terminal coupled to other terminal of the primary coil and a current output terminal coupled to a primary side ground level through a current detection resistor;
a control transistor having a base coupled to the current output terminal, a collector coupled to a control terminal of the switching element and an emitter coupled to the primary side ground level; and
an error detection circuit for detecting an error voltage of a DC output obtained by rectifying and smoothing an output of the secondary coil, wherein
the control transistor controls a switching operation of the switching element based on the error voltage thereby to stabilize a voltage of the DC output and limits a current flowing into the switching element within a predetermined range of value based on a voltage between both terminals of the current detection resistor, the switching power supply is arranged in that
when a value obtained by dividing a resistance value by a rated watt is defined as a broken index,
a breaking resistor is inserted at an arbitrary portion of a current path from an output point of the primary side positive power supply to the current input terminal,
a broken index of the breaking resistor is set in a range of two times to four times as large as a broken index of the current detection resistor, and
each of the breaking resistor and the current detection resistor is formed by a metal film resistor.
That is, a current of the same value always flows through the breaking resistor and the current detection resistor. Further, the broken index represents a degree how a resistor likely open-broken when an over-current flows through the resistor. Thus, when a short-circuit occurs at the primary coil or the switching element and so an over-current flows into both the breaking resistor and the current detection resistor, the open-breakage occurs at the breaking resistor side but does not occur at the current detection resistor side. Further, since each of the breaking resistor and the current detection resistor is the same kind of resistor, when an over-current of the same value flows into these resistors, the resistor with a larger broken index is necessarily open-broken. Thus, the resistance value and the rated watt of each of the breaking resistor and the current detection resistor are determined so that a broken index of the breaking resistor becomes larger than that of the current detection resistor. Further, when a broken index of the breaking resistor is set in a range of two times to four times as large as that of the current detection resistor, since the resistance value of the breaking resistor does not become too large, the dissipation power of the breaking resistor can be suppressed to a small value.
The switching power supply according to the invention is arranged in a manner that in the switching power supply including:
a transformer around which a primary coil and a secondary coil are wound, one terminal of the primary coil being introduced to a primary side positive power supply;
a switching element having a current input terminal coupled to other terminal of the primary coil and a current output terminal coupled to a primary side ground level through a current detection resistor;
a control transistor having a base coupled to the current output terminal, a collector coupled to a control terminal of the switching element and an emitter coupled to the primary side ground level; and
an error detection circuit for detecting an error voltage of a DC output obtained by rectifying and smoothing an output of the secondary coil, wherein
the control transistor controls a switching operation of the switching element based on the error voltage thereby to stabilize a voltage of the DC output and limits a current flowing into the switching element within a predetermined range of value based on a voltage between both terminals of the current detection resistor, the switching power supply is arranged in that
when a value obtained by dividing a resistance value by a rated watt is defined as a broken index,
a breaking resistor is inserted at an arbitrary portion of a current path from an output point of the primary side positive power supply to the current input terminal, and
a broken index of the breaking resistor is set to be larger than a broken index of the current detection resistor.
That is, a current of the same value always flows through the breaking resistor and the current detection resistor. Further, the broken index represents a degree how a resistor likely open-broken when an over-current flows through the resistor. Thus, when a short-circuit occurs at the primary coil or the switching element and so an over-current flows into both the breaking resistor and the current detection resistor, the open-breakage occurs at the breaking resistor side but does not occur at the current detection resistor side.
In addition to the aforesaid configuration, each of the breaking resistor and the current detection resistor is formed by a metal film resistor. That is, since each of the breaking resistor and the current detection resistor is the same kind of the resistor, when an over-current of the same value flows into these resistors, the resistor with a larger broken index is necessarily open-broken. Thus, the resistance value and the rated watt of each of the breaking resistor and the current detection resistor are determined so that a broken index of the breaking resistor becomes larger than that of the current detection resistor.